In an echo of Richard Nixon’s Saturday Night Massacre in 1973, President Trump abruptly fires FBI Director James Comey. Then, in a television interview, Trump fuels cover-up allegations by linking the ouster of Comey to a pending investigation into Russian interference in the 2016 presidential election. Trump then warns Comey not to leak because there may be White House “tapes” of their meetings.

For excitable Democrats, it was enough to leap from Watergate to talk of impeachment. “We’re actually pretty close to considering impeachment,” Kentucky Democratic Representative John Yarmuth told a home-state television station after Comey’s firing. In fact, impeachment proceedings probably aren’t imminent. Republican leadership in the House and Senate will see to that. But we’re a big step closer after credible reports indicate that in February Trump asked Comey to drop an FBI investigation into former national security adviser Michael Flynn.

“I think we’re in impeachment territory now for the first time,” said David Gergen, a former White House aide to both Democrats and Republicans, including Nixon, on CNN. And Florida Democratic Representative Ted Deutch wrote on Twitter: “Asking FBI to drop an investigation is obstruction of justice. Obstruction of justice is an impeachable offense.”

What would impeachment actually take? It’s a two-step process. If the House of Representative impeaches—or charges—a president, then the Senate holds a trial and either acquits or convicts. Any House member can start the process by alleging the president has committed “treason, bribery, or other high crimes and misdemeanors.” A simple majority of the House can approve an article of impeachment; two-thirds of the Senate must vote to convict. In a presidential impeachment trial, the chief justice of the Supreme Court presides.

A central question is what sort of crime, aside from treason and bribery, constitutes a high crime or misdemeanor. Broadly, what the authors of the Constitution had in mind were abuses of official authority. Obstruction of justice is a classic high crime. Federal criminal law is expansive on the topic. It punishes anyone who “corruptly or by threats of force, or by any threatening letter or communication, influences, obstructs, or impedes, or endeavors to influence, obstruct, or impede, the due administration of justice.”

The articles of impeachment against both Nixon and President Clinton included obstruction allegations. Nixon, accused of trying to stymie probes of the Watergate scandal, avoided impeachment by resigning in 1974. Clinton was impeached in 1998 for his testimony about the Monica Lewinsky scandal but acquitted in the Senate. (Nixon and Clinton faced Congresses controlled by the opposing party, underscoring that, unless Democrats gain more power, impeachment of Trump remains a long shot.)

In federal court, proving obstruction beyond a reasonable doubt can be tough. That’s in large part because the law requires the defendant to have acted “corruptly,” a reference to the person having provable criminal intent. According to reports about the Comey memo describing his Feb. 14 White House meeting with Trump, the president asked Comey to drop his investigation of Flynn because the former national security adviser “is a good guy” who didn’t do anything wrong. Before broaching the subject, Trump is said to have ushered other administration officials out of the Oval Office so he could speak to Comey privately.

FILE PHOTO – Protesters gather to rally against US President Donald Trump’s firing of Federal Bureau of Investigation (FBI) Director James Comey, outside the White House in Washington May 10, 2017. (REUTERS/Jonathan Ernst/File Photo)

Despite the hint of secrecy, a skilled defense lawyer could argue in court that Trump wasn’t twisting Comey’s arm so much as he was expressing an opinion. Likewise, when Trump said, “I hope you can let this go,” he was being aspirational, not thuggish, his defense attorney would maintain. Unless there actually are White House tapes with Trump saying he wants to shield Flynn to protect himself, a criminal prosecution for obstruction might founder.

But an impeachment doesn’t take place in court; it happens before Congress. It’s an exercise of political judgment—how severely the president is damaging American democracy—more than an exercise in meeting legal standards. High crimes and misdemeanors in the impeachment context “don’t have to satisfy all the technical aspects of an ordinary crime,” says Noah Feldman, a Harvard law professor and Bloomberg View columnist. High crimes describes abuse of power that undermines the rule of law. “Using the presidential office to try to shut down the investigation of a senior executive official who was also a major player in the president’s campaign is an obvious and egregious abuse of power”—in other words, a high crime.

The White House has denied the accounts of the memo, saying Trump “never asked Mr. Comey or anyone else to end any investigation involving General Flynn.” The idea that Comey made all this up strains credulity. We’d be in a better position to judge if the memo, and a paper trail of other memos he’s said to have written about troubling encounters with Trump, were made public.

That could happen soon. Representative Jason Chaffetz, a Utah Republican who chairs the House Oversight Committee, has demanded that by May 24 the FBI turn over all memos and other recordings related to communication between Comey and Trump. In a statement striking because it came from a conservative Republican, Chaffetz said: “If true, these memoranda raise questions as to whether the president attempted to influence or impede the FBI’s investigation as it relates to Lt. Gen. Flynn.”

Trump’s attempt to stifle the Flynn probe is, of course, not the only evidence of obstruction relevant to any eventual impeachment proceeding. More circumstantial evidence might include Trump’s firing Comey with Russia on his mind and the president’s enlisting Deputy Attorney General Rod Rosenstein to provide contradictory written justification for the firing. There’s also the president’s unlikely sounding public claim that Comey told him on three occasions that Trump himself wasn’t under investigation.

House Speaker Paul Ryan has backed the call for all the Comey memos to be turned over to Congress, but that’s not to say that Republicans have the desire or courage to begin the impeachment process. Still, if the Comey memos provide yet more proof of Trump abuses, Democrats could start organizing for a potential removal of the president. If the stranger-than-fiction Trump presidency continues on the “downward spiral” described recently by Tennessee Republican Senator Bob Corker, the 2018 elections could result in a change of congressional control, making impeachment much more plausible.

In the meantime, impeachment preparation could be accelerated by the appointment on May 17 of a special counsel to oversee the FBI investigation into Russian election interference. Rosenstein announced the choice of Robert Mueller, a former FBI director, to ride herd on the probe. Mueller will answer to Rosenstein and ultimately to Trump, but he’ll presumably have more autonomy than an ordinary Justice Department prosecutor. And whatever he digs up could one day find its way into impeachment proceedings.

The bottom line: The road to an impeachment proceeding is long and complicated, and it has more to do with politics than legal tests.